Coaxial cable loop antenna



y 1944 w. s. H. FINCH ETAL COAXIAL CABLE LOOP ANTENNA Filed Aug; 13, 1941 JIVNVEN'ITOR 'i" 4 42%; ATTORNEY.

Patented May 16, 1944 COAXIAL CABLE LOOP ANTENNA William G. H. Finch, Newtown, Conn, and James M. Wakefield, Waldwick, N. J., assignors to Finch Telecommunications, Inc., Passaic, N. J., a corporation of Delaware Application August 13, 1941, Serial No. 406,576

(c asm-s3) 2 Claims.

. wire, the terminals of which are suitably joined to a tuning condenser or tuning circuit. The

problem of loop construction is not particularly formidable if the loop is to be utilized at normal broadcast frequencies; that is, of the order of one thousand kilocycles per second, since leakage of;

high frequency alternating current across insulating material is notpresent to great extent.

Thus it is possible to fabricate a loop by employing a circular Bakelite form and winding a number of turns thereupon. An ordinary loop will, if employed in direction finding apparatus,

introduce certain errors into the determination of the bearing. Thus it is well understood that false bearings may be obtained unless theloopis electrostatically balanced with respect to ground potential.

Unbalance may be minimized if symmetrical circuit arrangements are utilized with the loop. However, the most eflicient method of balancing the loop with respect to ground is by the application of an electrostatic shield. This shield may comprise a cylindrical metal enclosure curved to the form of the loop itself. This metal therefore forms'a housing for the loop and if grounded insures that all parts of the loop will have the same capacity with respect to ground, and will remain thus irrespective of neighbouring metallic objects of the orientation. of the loop.

' This housing must necessarily be broken by an insulating bushing in order that the shield does not act as a closed or short-circuited turn.

As disclosed in copending application Serial No. 377,436, filed February 5, 1941, of Thomas S. Leaser, Patent No. 2,263,972, dated Nov. 25, 1941, the loop may befabricated upona fiat Bakelite strip and the electrostaticcshield :may comprise an aluminum cylindrical housing. The upper portion :of this housing is .open'circuited to preclude the aforementioned short circuit effect and the shield is'grounded. As disclosed inthe aforementionedapplication, the aluminum housing is a cast or otherwise fabricated metallic cylinder curved to fit the .loop'itself. V v p It is obvious that the fabrication of uch .a loop jnvclvesa number of stages and.- due to the fact v tov that aluminum is a material relatively difficult to handle, may involve considerable expense.

Furthermore, at frequencies of the order to twenty-five megacycles per second, it would be highly impractical to utilize the flat Bakelite coil form for the loop itself. This is true since at such ultra-high frequencies, the leakage across ordinary insulators is excessive and reduces the Q of the loop considerably,

The Q of a coil is, as is well known, an index of the efiiciency of the coil in so far as leakage or resistance losses are concerned.

For accurate direction finding apparatus, the Q of a coil should be as high as is feasible and thus the very best insulating materal should be employed in conjunction therewith.

It is evident that at the ultra-high frequencies, the inductance of the loop required. for proper reception of the signal is extremely low and thus since the inductance of a. coil varies substan- .tially as the square of the number of turns, the

number of turns will-be very small. In fact, a loop which may cover the ultra-high frequency range may normally have no more than a single or two turns of wire.

In fabricating such a loop in the manner described in the aforementioned copending application, a hat Bakelitestrip would be inserted into an electrostatic copper shield comprising a metallic housing. An operatoron the loop winding machine would then wind the necessary one or two turns-upon the Bakelite form to complete the loop.

Our invention contemplates a loop and a method for fabricating the same which is extremely efficient insofar as economy in manufacture and losses of an electrical nature are concerned.

More specifically, We have discovered that we may fabricate a loop for direction fiinding or similar apparatus from commercially available, preformed, coaxial cable which will provide a completed loop of an exceptionally high Q and which may be manufactured with a minimum Commercial coaxial cable, asis wellknown, comprises essentially ,a central conductor and an outer shield concentrically spaced therefrom by a plurality of equi-distantly spaced insulatin washers or the like. Since a relatively small amount of insulation is required with such a cable, good quality insulation may be utilized at a relatively low cost, Thus, for instanca i-fpolystyrene beads are utilized, the losses within this The outer conductor which now comprises the shield of my novel loop antenna is grounded and in order to prevent the shield from acting as a short-circuited turn to preclude the reception of signals, I remove a section of the outer conductor at one portion of the loop so as to introduce an air gap in the shield.

This method described, of course, indicates that the loop will have only one turn. However, as previously mentioned, one turn is sufiicient for loops required for the ultra-high frequency range. The Q of such a loop is extremely high inasmuch as relatively little insulating material of a solid nature is utilized to space the center wire and the outer shield.

Insulation such as polystyrene beads results in a very low loss loop. Furthermore, it should be noted that the remainder of the insulation between the central wire and the outer shield is air which comprises an excellent dielectric and the losses even at extremely high frequencies are relatively low.

It is therefore an object of our invention to provide for a relatively efiicient novel loop antenna.

A further object of our invention is to provide for a loop of extremely high Q formed from preformed shielded wire.

A still further object of our invention is to provide for a loop for use in connection with direction finding equipment fabricated from coaxial cable of a commercially available type having a central conductor spaced from an outer conducting shield by a plurality of insulating beads disposed along the central conductor.

Another object of our invention is to provide an electrostatically balanced loop utilizing coaxial cable bent to the proper form and having the outer shield thereof broken at one point to provide an insulating air gap.

These and other objects of our invention will now become apparent from the following specification taken in connection with the accompanying drawing, in which:

Figure 1 is a plan view of our novel loop in one phase of the construction thereof; and

Figure 2 is a cross-sectional view of the loop illustrated in Figure 1 showing the completed construction and the electrical connections thereof.

In the fabrication of the loop illustrated in Figures 1 and 2, the diameter thereof is first predetermined to meet the particular application thereof.

If the loop is to be circular, the perimeter thereof is then determined and a corresponding length of coaxial cable is obtained.

The coaxial cable required may be several inches greater than the actual perimeter of the coil for reasons which will be explained in a later paragraph.

The cable is then bent in any suitable manner obtained in the form of hollow copper tubing having a central parallel wire suitably supported therein.

There are various known methods for bending 5 copper tubing into circular forms and coils, for application other than that disclosed. Thus it is well known that machines may be obtained for bending copper tubing for use in connection with boiler heating coils and the like. Many of these machines are of relatively simple construction and thus may be inexpensively applied to the present needs for loops.

Accordingly, as illustrated in Figure 1, a loop of copper coaxial cable is formed having an outer -shielcl 2i and an inner concentric conductor 22 suitably spaced from the outer conductor.

Since the loop is normally utilized in connection with radio direction finding application wherein a series of interchangeable loops are desirable, our novel loop is mounted upon an adapter which permits the adaptation of this loop to conventional receptacles.

A metallic cup'23 is utilized to support the loop by securing the ends thereof. The cup 23 is in 5 turn supported upon an insulating member 2| which may preferably have a plurality of connecting prongs 25 extending therefrom.

In Figure 1, our novel loop antenna is shown in one phase of the construction thereof. Thus the entire coaxial cable has been bentto form a single turn and is joined to the supporting member 23. Actually in the construction of shielded loops, it is necessary to insert an insulated junction in the outer shield 2| in order to preclude the action of the shield as a short circuited turn.

Thus after the loop is assembled as illustrated in Figure 1, a section of the outer cylindrical conductor 22 may be removed to expose the inner 4 'wire. This accordingly open circuits the outer shield.

Referring now to Figure 2, the assembled loop is shown incross section. In order to provide a rigid support for the loop, the cup 23 is perforated at 26 and 21 which perforations are axially aligned.

The ends 3| and 32 of the outer conductor 2| -of the coaxial cable utilized for the loop are inserted into the openings 21 and 26 respectively and are suitably fastened therein.

As illustrated in Figures 1 and 2, the outer conductor may be fastened to the supporting member 23 by soldering the junctions 33 and 34. This serves the dual purpose of securing the outer conductor of the coaxial cable to the cup 23and precluding the entry of moisture or other=foreign particles into the hollow cylindrical conductor.

The inner conductor of a coaxial cable is nor- ..mally a highly conductive wire, such as copper, and is centrally supported therein by means of "a plurality'of regularly spaced insulating means or thelikefl These insulating members as is well knownin .the'art .of constructing coaxial cable,.iare sup- .ported'upon the central conductor during-'the fabrication thereof, and the outer concentric conductor is secured thereover.

The spacing between the inner insulating members will be determined by the particular application of the coaxial cable purchased,

'The'insula'tors utilized to centralize the inner conductor and insulatedly space it from the outer conductor are normally of an extremely low-loss,

to a circular loop of the proper diameter as illustrated in the figures. Coaxial cable is normally high dielectric material. 1 In selecting the particular" dielectric material to be utilized, it is important to consider that at ultra-high frequencies, the losses in dielectrics are normally due to conduction across the surface thereof, and not conduction throughout the volume. Thus it is necessary to select the dielectric material having a particularly low surface leakage :coefiicient.

As is well known, coaxial cables may be'pur chased'rolled upon reels and are constructed so that the cur-v-ature' thereof normally has no .efi'ect upon the spacing between inner and outer :oonductors, and normally does not affect the positionof the insulating members.

As illustrated :in Figure '2, the insulating members comprise beads .orthe like 4|, suitably per.- forated at 42 and supported :upon the inner conductor in the well known :manner. The insulating beads 41 are normally press-fitted upon "the inner conductor and thus lateral motion thereof is-avoided. Immediately subsequent r-to thelbending of the coaxial cable in to form a loop illustrated :in Figures El and :2 "and immediately prior to the insertion into the perforation 2.6., '21 of the cup 23, an insulating sleeve 143 :is slipped lover the outer conductor 2i. The sleeve may be iia'br icated from a suitable insulating material and has a curvature which equals that :oi the loop and an inner diameter which corresponds sub stantially to the outerdiameter \of the coaxial cable utilized for the loop. The material used for the sleeve 43 .must -'be of extremely low-floss since a poor insulator would 'efiectiyely ishort :circuit the opening the outer conductor of the cable at high frequencies. A suitable insulator for these high frequencies may for example be polymerized :styrene which in liquid form may be applied in several coats to a sleeve 43. The insulating support 23 is preferably of a similar material.

The pair of ring clamping members 44 and 45 are suitably positioned upon the insulating sleeve 43 as illustrated in Figures 1 and 2 and comprise bands of metal having lugs 46 and 41 extending from each end thereof. These lugs are perforated at and 52 and a pair of bolts and nuts are utilized to cause these clamping members 44 and 45 to bear tightly against the sleeve 43.

In the fabrication of the loop the sleeve 43 is permitted to remain free upon the outer conductor 2| and the clamping bolts and nuts to be described are not used until the loop is completed as indicated in Figure 2. g

The length of coaxial cable required to fabricate the loop is greater actually than the circumference of the circular loop itself, since it is necessary to provide extensions of the inner conductor to join the loop to a suitable radio circuit.

To this end, we use a length of coaxial cable which is somewhat greater than the circumference of the loop, and remove a section of the outer concentric shield at each end thereof to provide the extensions 55 and 56 of the inner conductor which are suitable for joining into an associated circuit.

As previously mentioned, the member 23, which is preferably of metal, is used to support the outer shield 2| of the loop.

The member 23 is supported upon a suitable insulating post 24 as previously described which, as indicated in Figures 1 and 2, provides the extending prongs 51 and 58 for joining to a suitable corresponding receptacle. The prongs 51 and 58 may be formed in a well known manner and may comprise hollow cylindrical members having a position circular flange BI and may be inserted into corresponding cylindrical openings -62 and '63 within the insulatmg member 24.

The upper :endst614 and 65 are @flai'erl :by a suitable punch so that the :prongs '51 asd 53 remain positioned within the insulating support .24.

Theshoulderifi is out into'theinsulatingzmember 245 that it may be press fitted at it in (the metallic supporting member '23, as illustrated in Figure 2.

.During the construction of the loop, the ends 3| and '32 are inserted into the corresponding openings .24 and 26. The extensions 515 and 5-6 of the inner conductorare brought through the supporting :member 23 and through the passages 62 and 63 of :tlie insulator '24 and are inserted into the brongs :58 and 5.1.

The .ends .o-fthe inner conductor are then :snit ably secured to the prongs :5! and 58 as by-solder- .As previously mentioned, it is essential *to pre- :cludethe action of the shield as ashort-ci-rcuited turn to prevent the reception of a signal by the inner conductor. It is accordingly necessary :to break the shield and insert an insulating support therefor.

As indicated in Figure 2, the upper end of the :outer concentric :shield 111315 been out and a :sectionthereoi removed to expose the ends H and 12 thereof. However, it is important toxnote that the inner conductor has not been damaged in any :manner.

Since normally a loop antenna of st-he :type Ell:- sl-ustrated is utilized :ior outdoor work, the-insulating sleeve -43 is slipped orer the opening the outer shield *betweenthe :endS "TI and it? and the, clamping members '44 and 45 are ri idly so cured thereto.

7 This is accomplished by tightening the nuts and bolts 13 and 14 so that the clamps bring the insulating member 43 to bear in a manner which precludes the entry of moisture and other foreign substances.

It is well known in the art of radio direction finding loop antennae, that a shielded loop may be formed by opening the shield at any point. However, it is essential to have a balanced loop for proper radio direction finding equipment and thus the shield is normally opened at a point symmetrically disposed with respect to the ends thereof. However, it is possible to open the shield at the lower end thereof. This may be accomplished in a most expeditious manner since it is merely necessary to form the supporting member 23 of an insulating material and join the ends of the loop 3| and 32 thereto in any suitable manner. It is thus possible to fabricate a balanced shielded loop without introducing the operation of cutting the outer coaxial shield at H and 12 as illustrated in Figure 2.

If it is necessary to properly position the loop illustrated in Figure 2 within a corresponding receptacle, a locating pin 8| may be secured to the supporting member 23.

It will now be evident that the loop illustrated in Figures 1 and 2 comprises merely a single turn shielded to preclude deleterious effects of nearby metallic objects and other interfering signals as has been described. Since the inductance of a one-turn loop is extremely small, the loop indicated is particularly adaptable to the reception of ultra-high frequency signals.

For this application, the loop illustrated is particularly efficient since the insulating beads 4| normally inserted into coaxial cable are of lowloss material. Furthermore, as will be noted the inner conductor and the outer concentric shield are practically spaced entirely by air which as is well known forms an extremely suitable dielectric medium for such purposes. The actual number of insulating washers 4| used to separate inner and outer conductors will be determined by the particular type of insulating material employed, the type of coaxial cable used, andthe diameter of the loop to be formed.

The method illustrated and described for fabricating single turn shielded loops may be extended readily to loops having several turns. Thus it is merely necessary to form a coil of a number of turns of coaxial cable and open-circuit the outer concentric conductor 2| at one point in each turn thereof. The ends of the coaxial cable used for the coil may then be brought to a support similarto that illustrated in connection with Figures 1 and 2. This support which may be similar to the members 23 and 24 will serve to support the turns of the loop in a fixed space'relation and to carry the prongs for joining into another circuit.

Also, commercially available coaxial cable having a number of strands within an outer sheath I maybe employed. These strands are insulated from each other by spaces as in the single strand cable. By suitably interconnecting the ends of the strands within the sheath a loop antenna having a plurality of turns may be formed. A section of the sheath may be cut away, and a base provided.

Inasmuch as we have described and illustrated only one modification of a loop antenna for radio direction finding, many other variations thereo'f willnow be present to those skilled in the art.

Therefore, we prefer not to be bound by the specific disclosures hereinabove set forth, but only by the spirit and scope of the appended claims.

We claim:

1. A shielded loop antenna comprising a preformed wire, said wire having an inner conductor and an outer cylindrical metallic hollow circular shield, equi'distantly spaced insulation washers for supporting said inner conductor concentrically with respect to said outer conductor, a metallic cup for supporting said shield having concentric perforations at its sides for securing the ends of said shield and through which the ends of the inner conductor pass into said cup, an insulation cap for closing said cup and having conductor prongs embedded therein, said ends of said inner conductor being secured to said conductor prongs.

' 2. The method of fabricating a loop antenna comprising the steps of bending a predetermined length of preformed coaxial shielded cable having an inner conductor mounted by disk insulators in air spaced relation in an outer conductive shield and constructed so that the curvature thereof has no effect upon the spacing between inner and outer conductors and does not affect the position of the insulating washers, positioning an insulating sleeve upon said shield, securing the ends of said shield to a supporting member, removing a section of said cable shield at a point symmetrically disposed with respect to the ends of said shield, and joining the endsof said inner conductor to a connector.

WILLIAM G. H. FINCH. JAMES M. WAKEFIELD. 

